Semiconductor device using polymer-containing photoresist, and process for manufacturing the same

ABSTRACT

The present invention relates to a semiconductor device using a copolymer-containing photoresist, and a process for manufacturing the same. As a norbornene derivative (monomer) having a hydrophilic group is synthesized and introduced to the backbone chain of a polymer, the polymer according to the present invention has excellent etching resistance and heat resistance, which are the characteristic points of alicyclic olefin structure, and provide excellent resolution due to prominent enhancement of adhesiveness resulted from introducing a hydrophilic group (—OH).

BACKGROUND OF THE INVENTION

The present invention relates to semiconductor devices using aphotoresist, and to processes for manufacturing the same.

Various types of photoresists have been used or proposed. These resistsshould have a variety of desirable characteristics or properties. Ingeneral, all or most of these resists generally demand etchingresistance, adhesiveness with low light absorption at 193 nm wavelengthfor ArF. Additionally, the resists should be developable by using 2.38wt % aqueous tetramethylammonium hydroxide (TMAH) solution. It is,however, difficult to synthesize a polymer satisfying one or all theseproperties.

Many researches have focused on studies on norbolac type resin as aresin to increase transparency at 193 nm wavelength and increase etchingresistance. As merely an example, “Bell Labs” tried to introducealicyclic unit to the backbone chain of a copolymer in order to enhanceetching resistance. A copolymer resin in which the backbone chain hasnorbornene, acrylate and maleic anhydride substituent, as represented bychemical formula I has been suggested:

[FORMULA I] See Appendix A

In the polymer resin of formula I, the maleic anhydride portion (portionA) was used for polymerizing alicyclic olefin group.

The maleic anhydride portion is soluble in 2.38% aqueous TMAH solutioneven it is not exposed, and thus a y-portion having tert-butylsubstituent should be highly increased in order to prevent dissolution.But increase of the y-portion causes relative decrease of z portion,which enhances sensitivity and adhesiveness with substrate, to causedisadvantage in that photoresist is removed from the wafer in practicalpatterning.

Thus, an effective pattern cannot be formed without separately using asolubility controlling agent, and even if a pattern is formed by using asolubility controlling agent, the adhesiveness is too poor to be appliedto practical patterning.

Under such circumstances, Bell Labs tried to solve the above-mentionedproblems by using a solubility controlling agent of cholesterol type andby employing two-component resist comprising a polymer of cyclo-olefinand maleic anhydride.

However, in this case, very large amount (about 30% by weight based onthe polymer) of the solubility controlling agent should be used, andthus the polymer of the above molecular structure basically has too lowreproducibility and too high cost to be used as a polymer for aphotoresist. From the above, it is seen that an improved photoresistresin that is cost effective, easy to manufacture, and has desirableother properties is clearly desired.

SUMMARY OF THE INVENTION

The present inventors have performed intensive studies to overcome theabove limitations encountered in conventional resins, and as a result,they could synthesize novel norbornene derivatives having hydrophilicgroup(s). In a specific embodiment, the present invention provides amethod using a step of introducing the monomer to the backbone chain ofthe polymer to develop a polymer having excellent resolution due toprominent enhancement of adhesive strength by introducing a hydrophilicgroup (—OH). The present method yields a photoresist having excellentetching resistance and heat resistance which are the characteristics ofalicyclic olefins.

Numerous benefits or advantages are achieved by way of the presentinvention over conventional techniques. In a specific embodiment, thepresent invention provides a monomer comprising a novel norbornenederivative represented by following formula II:

[FORMULA II] See Appendix A

wherein, R′ and R″ independently represent hydrogen, or linear orbranched C₁-C₄ alkyl group with or without substituent(s), m representsnumber of 1 to 8, and n represents number of 1 to 6, and a process forpreparing the same.

In an alternative embodiment, the present invention provides a polymerfor a photoresist comprising bicycloalkene compounds represented bychemical formulas II and V, and maleic anhydride of chemical formula VI,and a process for preparing the same.

In a further embodiment, the present invention provides a polymer forphotoresist represented by formula III or IV, which comprisesbicycloalkene compound(s) and maleic anhydride, and process forpreparing the same.

[FORMULA III] See Appendix A

[FORMULA IV] See Appendix A

In the formula, R′ and R″ independently represent hydrogen, or linear orbranched C₁-C₄ alkyl group with or without substituent (s), R₁ and R₂independently represent hydrogen, or linear or branched alkyl,cycloalkyl, alkoxyalkyl or cycloalkoxyalkyl having 1 to 10 carbon atomswith or without substituent(s), m represents number of 1 to 8, and theratio x: y: z is (0-99%) : (0-99%) : (0-99%) [provided that x is0.005-0.9 part by mole, and y and z are 0.001-0.9 part by mole,respectively, in case of formula IV].

In still a further embodiment, the present invention provides a polymerfor photoresist which comprises bicycloalkene compounds represented byformulas II and V, and maleic anhydride represented by formula VI, and aphotoresist formed by using a polymer represented by formula III, IV,VII, VIII or IX, and a process for manufacturing the photoresist.

[FORMULA V] See Appendix A

[FORMULA VI] See Appendix A

Still further, the present invention provides a polymer for photoresist.The polymer includes a variety of elements such as bicycloalkenecompounds represented by formulas II and V, and maleic anhydriderepresented by formula VI, and a process for forming a photoresistpattern by the use of the photoresist formed with the polymerrepresented by formula III, IV, VII, VIII or IX.

Moreover, the present invention provides a polymer for photoresist. Thepresent polymer includes a variety of elements such as bicycloalkenecompounds represented by formulas II and V, and maleic anhydriderepresented by formula VI, and a semiconductor device using thephotoresist formed with the polymer represented by formula III, IV, VII,VIII or IX.

[FORMULA VII] See Appendix A

[FORMULA VIII] See Appendix A

[FORMULA IX] See Appendix A

BRIEF DESCRIPTION OF DRAWING

FIG. 1 illustrates NMR data of the polymer prepared in Example 9.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is embodied in a polymer usable for lithographyprocess using ultra-short wavelength light source such as KrF (248 nm),ArF (193 nm), X-ray, ion beam and E-beam which is expected to be appliedin 1G or 4G DRAM or other highly integrated circuits, and having a novelnorbornene monomer introduced to the backbone chain of the polymer. Theinvention is further embodied in a process for manufacturing the same,and a photoresist containing the same polymer.

Among the bicycloalkene compounds represented by formula II, preferablecompounds are 3-hydroxypropyl 5-norbornen-2-carboxylate, 4-hydroxybutyl5-norbornen-2-carboxylate, 5-hydroxypentyl 5-norbornen-2-carboxylate,6-hydroxyhexyl 5-norbornen-2-carboxylate, 7-hydroxyheptyl5-norbornen-2-carboxylate, 8-hydroxyoctyl 5-norbornen-2-carboxylate, orthe like.

The bicycloalkene derivatives (formula II) according to the presentinvention can be prepared by reacting hydroxyalkyl acrylate withcyclopentadiene in the presence of tetrahydrofuran.

The hydroxyalkyl acrylate is preferably selected from a group consistingof 3-hydroxypropyl acrylate, 4-hydroxybutyl acrylate, 5-hydroxypentylacrylate, 6-hydroxyhexyl acrylate, 7-hydroxyheptyl acrylate and8-hydroxyoctyl acrylate.

The polymer according to the present invention (formula III or IV) canbe prepared by polymerizing the bicycloalkene compounds represented bychemical formulas II and V and maleic anhydride represented by formulaVI in the presence of polymerization initiator.

Preferable bicycloalkenes to be used for the polymers for photoresistaccording to the present invention may be one or more compounds selectedfrom a group consisting of bicycloalkenes represented by formula Vwherein R is hydrogen or tert-butyl group, and bicycloalkenesrepresented by formula II wherein m is 3 and R′ and R″ are hydrogen.

More preferably, bicycloalkenes for the polymers according to thepresent invention are selected from a group consisting of3-hydroxypropyl 5-norbornen-2-carboxylate, tert-butyl5-norbornen-2-carboxylate, 5-norbornen-2-carboxylic acid,3-hydroxypropyl bicyclo[2,2,2]oct-5-en-3-carboxylate, tert-butylbicyclo[2,2,2-oct-2-en-carboxylate andbicyclo[2,2,2]oct-5-en-2-carboxylic acid.

Among the polymers according to the present invention, the polymersprepared from the bicycloalkene represented by formula V wherein R ishydrogen or tert-butyl and n is 1, the bicycloalkene represented byformula II wherein R′ and R″ are hydrogen and m is 3 [i.e., one or morebicycloalkenes selected from a group consisting of 3-hydroxypropyl5-norbornen-2-carboxylate, tert-butyl 5-norbornen-2-carboxylate and5-norbornen-2-carboxylic acid], and maleic anhydride represented byformula VI are particularly preferable.

The polymers according to the present invention can be prepared by aconventional polymerization process such as bulk polymerization orsolution polymerization. Polymerization initiators usable in the presentinvention include benzoyl peroxide, 2,2′-azobisisobutyronitrile (AIBN),acetyl peroxide, lauryl peroxide, tert-butyl peracetate, di-tert-butylperoxide, or the like.

As a solvent, cyclohexanone, methyl ethyl ketone, benzene, toluene,dioxane, dimethylformamide and/or tetrahydrofuran may be usedindividually, or in a mixture.

In the process for preparing the polymers according to the presentinvention, general polymerization condition including temperature andpressure of radical polymerization may be controlled dependent upon theproperty of the reactants, but it is preferable to carry out thepolymerization reaction at a temperature between 60 and 200° C. for 4 to24 hours.

The polymers represented by formula III or IV according to the presentinvention have molecular weight of 3,000-100,000, and can be used inlithography process using ultra-short wavelength light such as KrF orArF light source, X-ray, ion beam or E beam, which is expected to beapplied to 1G or 4G DRAM.

The polymers according to the present invention may be used in theformation of a positive micro-image by preparing a photoresist solutionin which the polymer is mixed with an organic solvent and a conventionalphoto acid generator according to a conventional process for preparing aphotoresist composition.

In the process for forming photoresist pattern of semiconductor element,the amount of the polymer according to the present invention depends onthe organic solvent or photo acid generator used, and the condition oflithography, but conventionally it is about 10 to 30% by weight on thebasis of the organic solvent used in the preparation of the photoresist.

The process for forming a photoresist pattern of a semiconductor elementby using the polymer according to the present invention is described indetail here-in-below:

The polymer according to the present invention is dissolved incyclohexanone, methyl 3-methoxypropionate, ethyl 3-ethoxypropionate,ethyl lactate or propylene glycol methyl ether acetate at aconcentration of 10 to 30% by weight. Onium salt or organic sulfonicacid as inorganic acid generator (0.01-10% by weight based on thepolymer) is incorporated to the solution, and the mixture is thenfiltered through an ultra-micro filter to prepare photoresist solution.

As the photo acid generator, triphenylsulfonium triplate,dibutylnaphthylsulfonium triplate, 2,6-dimethylphenylsulfonate,bis(arylsulfonyl)-diazomethane, oxime sulfonate and1,2-diazonaphthoquinon-4-sulfonate can be mentioned.

Then, the photoresist solution is spin-coated on a silicon wafer to forma thin film, which is then pre-baked in an oven at 80-150° C. or on ahot plate for 1-5 minutes, exposed to light by using far ultravioletexposer or an eximer laser exposer, and post-baked in an oven at atemperature between 100° C. and 200° C. or on a hot plate for 1 secondto 5 minutes.

The exposed wafer is impregnated in 2.38% aqueous TMAH solution for 30seconds to 1.5 minutes to obtain an ultra-micro positive photoresistpattern.

The syntheses of novel norbornene derivatives according to the presentinvention, the syntheses of polymers using the derivatives,manufacturing the photoresist comprising the polymers, and process forforming micro-patterns in a semiconductor device are described in detailby referring to Examples.

A better understanding of the present invention may be obtained in lightof following examples which are set forth to illustrate, but are not tobe construed to limit, the present invention.

Syntheses of Norbornen Derivatives EXAMPLE I

Synthesis of 3-hydroxypropyl 5-norbornene-2-carboxylate

In a reactor, cyclopentadiene (66 g) and tetrahydrofuran solvent (500 g)were charged, and the mixture was stirred homogeneously. To the reactionmixture, 3-hydroxypropyl acrylate (130 g) was added, and the resultantmixture was stirred at a temperature between −30° C. and 60° C. forabout 10 hours to carry out the reaction.

When the reaction was completed, the solvent was removed by using arotary evaporator, and the residue was distilled under reduced pressureto obtain 168 g (yield: 86%) of 3-hydroxypropyl5-norbornene-2-carboxylate.

EXAMPLE II

Synthesis of 4-hydroxybutyl 5-norbornen-2-carboxylate

The same procedure described in Example I was repeated but4-hydroxybutyl acrylate (144 g) was used instead of 3-hydroxypropylacrylate to give 178 g (yield: 85%) of 4-hydroxybutyl5-norbornen-2-carboxylate.

EXAMPLE III

Synthesis of 5-hydroxypentyl 5-norbornen-2-carboxylate

The same procedure described in Example I was repeated but5-hydroxypentyl acrylate (158 g) was used instead of 3-hydroxypropylacrylate to give 190 g (yield: 85%) of 5-hydroxypentyl5-norbornen-2-carboxylate.

EXAMPLE IV

Synthesis of 6-hydroxyhexyl 5-norbornen-2-carboxylate

The same procedure described in Example I was repeated but6-hydroxyhexyl acrylate (172 g) was used instead of 3-hydroxypropylacrylate to give 205 g (yield: 86%) of 6-hydroxyhexyl5-norbornen-2-carboxylate.

EXAMPLE V

Synthesis of 7-hydroxyheptyl 5-norbornen-2-carboxylate

The same procedure described in Example I was repeated but7-hydroxyheptyl acrylate (186 g) was used instead of 3-hydroxypropylacrylate to give 204 g (yield: 81%) of 7-hydroxyheptyl5-norbornen-2-carboxylate.

EXAMPLE VI

Synthesis of 8-hydroxyoctyl 5-norbornen-2-carboxylate

The same procedure described in Example I was repeated but8-hydroxyoctyl acrylate (200 g) was used instead of 3-hydroxypropylacrylate to give 207 g (yield: 78%) of 8-hydroxyoctyl5-norbornen-2-carboxylate.

Synthese of Bicycloalkene Compounds EXAMPLE VII

Synthesis of tert-butyl 5-norbornen-2-carboxylate

In a reactor, cyclopentadiene (66 g) and tetrahydrofuran solvent (500 g)were charged, and the mixture was stirred homogeneously. To the reactionmixture, tert-butyl acrylate (128 g) was added, and the resultantmixture was stirred at a temperature between −30° C. and 60° C. forabout 10 hours to carry out the reaction.

When the reaction was completed, the solvent was removed by using arotary evaporator, and the residue was distilled under reduced pressureto obtain 175 g (yield: 90%) of tert-butyl 5-norbornene-2-carboxylate.

EXAMPLE VIII

Synthesis of 5-norbornen-2-carboxylic acid

In a reactor, cyclopentadiene (66 g) and tetrahydrofuran solvent (500 g)were charged, and the mixture was stirred homogeneously.

To the reaction mixture, acrylic acid (72 g) was added, and theresultant mixture was stirred at a temperature between—30° C. and 60° C.for about 10 hours to carry out the reaction.

When the reaction was completed, the solvent was removed by using arotary evaporator, and the residue was distilled under reduced pressureto obtain 124 g (yield: 90%) of 5-norbornen-2-carboxylic acid.

Syntheses of Polymers EXAMPLE IX

Synthesis of poly[3-hydroxypropyl 5-norbornen-2-carboxylate tert-butyl5-norbornen-2-carboxylate/5-norbornen-2-carboxylic acid/maleicanhydride] polymer (Formula VII)

In tetrahydrofuran, benzene or toluene, dissolved were 3-hydroxypropyl5-norbornen-2-carboxylate (0.05-0.8 mol), tert-butyl5-norbornen-2-carboxylate (0.5-0.95 mol), 5-norbornen-2-carboxylic acid(0.01-0.3 mol) and maleic anhydride (1 mol).

Then, 2,2′-azobisisobutyronitrile (AIBN) (0.01-10 g), as apolymerization initiator, was added thereto, and the reaction wasperformed at a temperature between 60° C. and 70° C. for 4-24 hours.

Crude product thus obtained was precipitated from ethyl ether or hexane,and the precipitate was filtered and dried under reduced pressure togive poly[3-hydroxypropyl 5-norbornen-2-carboxylate/tert-butyl5-norbornen-2-carboxylate/5-norbornen-2-carboxylic acid/maleicanhydride] polymer represented by formula VII, of which the NMR data isshown in FIG. 1. (yield: ≧70%)

EXAMPLE X

Synthesis of poly[4-hydroxybutyl 5-norbornen-2-carboxylate/tert-butyl5-norbornen-2-carboxylate/5-norbornen-2-carboxylic acid/maleicanhydride] polymer (Formula VIII)

In tetrahydrofuran, benzene or toluene, dissolved were 4-hydroxybutyl5-norbornen-2-carboxylate (0.05-0.8 mol), tert-butyl5-norbornen-2-carboxylate (0.5-0.95 mol), 5-norbornen-2-carboxylic acid(0.01-0.3 mol) and maleic anhydride (1 mol).

Then, 2,2′-azobisisobutyronitrile (AIBN) (0.01-10 g), as apolymerization initiator, was added thereto, and the reaction wasperformed at a temperature between 60° C. and 70° C. for 4-24 hours.

Crude product thus obtained was precipitated from ethyl ether or hexane,and the precipitate was filtered and dried under reduced pressure togive poly[4-hydroxybutyl 5-norbornen-2-carboxylate/tert-butyl5-norbornen-2-carboxylate/5-norbornen-2-carboxylic acid/maleicanhydride] polymer represented by formula VIII (yield:≧70%).

EXAMPLE XI

Synthesis of poly[3-hydroxypropyl 5-norbornen-2-carboxylate/tert-butyl5-norbornen-2-carboxylate/mono-methylcis-5-norbornen-endo-2,3-dicarboxylate/maleic anhydride] polymer(Formula IX)

In tetrahydrofuran, benzene or toluene, dissolved were 3-hydroxypropyl5-norbornen-2-carboxylate (0.05-0.8 mol), tert-butyl5-norbornen-2-carboxylate (0.5-0.95 mol), mono-methylcis-5-norbornen-endo-2,3-dicarboxylate (0.01-0.3 mol) and maleicanhydride (1 mol).

Then, 2,2′-azobisisobutyronitrile (AIBN) (0.01-10 g), as apolymerization initiator, was added thereto, and the reaction wasperformed at a temperature between 60° C. and 70° C. for 4-24 hours.

Crude product thus obtained was precipitated from ethyl ether or hexane,and the precipitate was filtered and dried under reduced pressure togive poly[3-hydroxypropyl 5-norbornen-2-carboxylate/tert-butyl5-norbornen-2-carboxylate/mono-methylcis-5-norbornen-endo-2,3-dicarboxylate/maleic anhydride] polymerrepresented by formula IX (yield: 74%).

Preparation of Photoresist and Pattern Formation EXAMPLE XII

Poly[3-hydroxypropyl 5-norbornen-2-carboxylate/tert-butyl5-norbornen-2-carboxylate/5-norbornen-2-carboxylic acid/maleicanhydride] polymer (formula VII) (10 g) was dissolved in 3-methoxymethylpropionate (40 g, solvent), and triphenylsulfonium triplate ordibutylnaphthylsulfonium triplate (about 0.01-1 g) as a photo acidgenerator, was added thereto. After stirring, the mixture was filteredthrough a 0.1 μm filter to give a photoresist. Then the photoresist wascoated on a surface of a wafer. After heat treatment, the photoresistwas developed by a photo-developing process to form a pattern. Thus, asemiconductor element having perpendicular L/S pattern with thickness ofthe polymer 0.6 μm and the width of 0.13 μm was obtained.

EXAMPLE XIII

Poly[3-hydroxypropyl 5-norbornen-2-carboxylate/tert-butyl5-norbornen-2-carboxylate/mono-methylcis-5-norbornen-2-endo-2,3-dicarboxylate/maleic anhydride] polymer(formula IX) (10 g) was dissolved in 3-methoxymethyl propionate (40 g,solvent), and triphenylsulfonium triplate or dibutylnaphthylsulfoniumtriplate (about 0.01-1 g) as a photo acid generator, was added thereto.After stirring, the mixture was filtered through a 0.1 μm filter to givea photoresist. Then the photoresist was coated on a surface of a wafer.After heat treatment, the photoresist was developed by aphoto-developing process to form a pattern. Thus, a semiconductorelement having perpendicular L/S pattern with thickness of the polymer0.6 μm and the width of 0.13 μm was obtained.

As described above, the photoresist formed by using the polymers for KrFor ArF according to the present invention has excellent etchingresistance, heat resistance and adhesiveness, and is developable with2.38% aqueous TMAH solution, so that satisfactory results can beobtained in view of resolution of perpendicular L/S pattern of 0.13 μmwith resist thickness of 0.6 μm, and the depth of a focus.

Many modifications and variations of the present invention are possiblein light of the above teachings. Therefore, it is to be understood thatwithin the scope of the appended claims, the invention may be practicedotherwise than as specifically described.

What is claimed is:
 1. A polymer comprising poly[3-hydroxypropyl5-norbornene-2-carboxylate/tert-butyl5-norbornene-2-carboxylate/mono-methylcis-5-norbornene-endo-2,3-dicarboxylate/maleic anhydride] represented byformula IX:

wherein the molar ratio x:y:z is (0.1%-99%):(0.1%-99%):(0.1%-99%).
 2. Apolymer comprising a photoresist polymer represented by formula IV:

wherein R′ and R″ independently represent hydrogen, or a linear orbranched C₁-C₄ alkyl group with or without substituent(s); R₁ and R₂independently represent hydrogen, or linear or branched alkyl,cycloalkyl, alkoxyalkyl or cycloalkoxyalkyl having 1 to 10 carbon atomswith or without substituent(s); m represents a number of 1 to 8; and themolar ratio x:y:z is (0.005-0.9):(0.001-0.9):(0.001-0.9).
 3. Aphotoresist composition comprising: (a) a photoresist polymer, (b) asolvent, and (c) a photoacid generator; wherein said photoresist polymeris poly[3-hydroxypropyl 5-norbornene-2-carboxylate/tert-butyl5-norbornene-2-carboxylate/mono-methylcis-5-norbornene-endo-2,3-dicarboxylate/maleic anhydride] or a polymerof formula IV:

wherein R′ and R″ independently represent hydrogen, or a linear orbranched C₁-C₄ alkyl group with or without substituent(s); R₁ and R₂independently represent hydrogen, or linear or branched alkyl,cycloalkyl, alkoxyalkyl or cycloalkoxyalkyl having 1 to 10 carbon atomswith or without substituent(s); m represents a number of 1 to 8; and themolar ratio x:y:z is (0.005-0.9):(0.001-0.9):(0.001-0.9).
 4. Thephotoresist composition of claim 3, wherein the amount of saidphotoresist polymer present in said composition is between 10 and 30% byweight based on the amount of said solvent.
 5. The photoresistcomposition of claim 3, wherein said solvent is selected from the groupconsisting of cyclohexanone, methyl 3-methoxypropionate, ethyl3-ethoxypropionate, ethyl lactate, propylene glycol methyl etheracetate, and mixtures thereof.
 6. The photoresist composition of claim3, wherein said photoacid generator is an onium salt or an organicsulfonic acid.
 7. The photoresist composition of claim 3, wherein theamount of said photoacid generator present in said composition isbetween 0.01 and 10% by weight based on the amount of said photoresistpolymer.
 8. A process for producing a photoresist composition, saidprocess comprising the steps of: (a) adding a photoresist polymer and aphotoacid generator to a solvent to form a photoresist solution; and (b)filtering said photoresist solution to form said photoresistcomposition, wherein said photoresist polymer is poly[3-hydroxypropyl5-norbornene-2-carboxylate/tert-butyl5-norbornene-2-carboxylate/mono-methylcis-5-norbornene-endo-2,3-dicarboxylate/maleic anhydride] or a polymerof formula IV:

R′ and R″ independently represent hydrogen, or a linear or branchedC₁-C₄ alkyl group with or without substituent(s); R₁ and R₂independently represent hydrogen, or linear or branched alkyl,cycloalkyl, alkoxyalkyl or cycloalkoxyalkyl having 1 to 10 carbon atomswith or without substituent(s); m represents a number of 1 to 8; and themolar ratio x:y:z is (0.005-0.9):(0.001-0.9):(0.001-0.9).
 9. The processof claim 8, wherein the amount of said photoresist polymer present insaid composition is between 10 and 30% by weight based on the amount ofsaid solvent.
 10. The process of claim 8, wherein said solvent isselected from the group consisting of cyclohexanone, methyl3-methoxypropionate, ethyl 3-ethoxypropionate, ethyl lactate, propyleneglycol methyl ether acetate, and mixtures thereof.
 11. The process ofclaim 8, wherein said photoacid generator is an onium salt or an organicsulfonic acid.
 12. The process of claim 8, wherein the amount of saidphotoacid generator present in said composition is between 0.01 and 10%by weight based on the amount of said photoresist polymer.